Pillar Modularity in fsc Topology Hybrid Ultramicroporous Materials Based upon Tetra(4-pyridyl)benzene.

Hybrid ultramicroporous materials (HUMs) are porous coordination networks composed of combinations of organic and inorganic linker ligands with a pore diameter of <7 Å. Despite their benchmark gas sorption selectivity for several industrially relevant gas separations and their inherent modularity, the structural and compositional diversity of HUMs remains underexplored. In this contribution, we report a family of six HUMs (SIFSIX-22-Zn, TIFSIX-6-Zn, SNFSIX-2-Zn, GEFSIX-4-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn) based on Zn metal centers and the tetratopic N-donor organic ligand tetra(4-pyridyl)benzene (tepb). The incorporation of fluorinated inorganic pillars (SiF6 2-, TiF6 2-, SnF6 2-, GeF6 2-, ZrF6 2-, and TaF7 2-, respectively) resulted in (4,6)-connected fsc topology as verified using single-crystal X-ray diffraction. Pure-component gas sorption studies with N2, CO2, C2H2, C2H4, and C2H6 revealed that the large voids and narrow pore windows common to all six HUMs can be leveraged to afford high C2H2 uptakes while retaining high ideal adsorbed solution theory (IAST) selectivities for industrially relevant gas mixtures: >10 for 1:99 C2H2/C2H4 and >5 for 1:1 C2H2/CO2. The approach taken, systematic variation of pillars with retention of structure, enables differences in selectivity to be attributed directly to the choice of the inorganic pillar. This study introduces fsc topology HUMs as a modular platform that is amenable to fine-tuning of structure and properties.

Introduction

[Zn(4,4′-bipyridine)2(SiF6)] (SIFSIX-1-Zn), reported in 1995,[1] is the prototypal hybrid coordination network (HCN) and is the parent of what is today a broad and growing platform of porous materials with a diverse range of topologies, pore sizes, pore chemistries, and properties.[2] That both the pore size and pore chemistry of HCNs are amenable to fine-tuning through crystal engineering approaches is a consequence of their inherently modular nature, which comes from their typical compositions: a divalent metal ion node, neutral N-donor linker ligand, and inorganic anion pillar.[2] The resulting hybrid coordination networks are thereby composed of geometrically simple components.[3] The ready availability of N-donor linker ligands of varying lengths (e.g., pyrazine, 2.8 Å; 4,4′-bipyridine, 7.1 Å; N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide, 15.4 Å) and inorganic dianions that can serve as linkers that offer strong electrostatics (e.g., SiF62–, GeF62–, AlF52–, NbOF52–, MoO42–, Cr2O72–) has been exploited to fine-tune properties that are of relevance to several important industrial separation challenges.[4−11] In particular, HCNs with pore diameters <7 Å (hybrid ultramicroporous materials, HUMs) exhibit selectivity values that are in some cases orders of magnitude greater than previous benchmark porous materials: SIFSIX-18-Ni-β, NbOFFIVE-1-Ni, and TIFSIX-3-Ni for direct air capture of CO2; SIFSIX-14-Cu-i for removal of acetylene from ethylene; and CROFOUR-1-Ni for Xe/Kr separation.[12−22]

Although crystal engineering has enabled systematic access to platforms of HUMs with exceptional properties, there is limited topological diversity among the HUMs reported thus far. The majority of HCNs, including HUMs, are pcu topology nets composed of octahedral metal centers linked by two ditopic N-donor linker ligands and a ditopic inorganic pillar anion. Among the non-pcu topology HCNs, only those that exhibit mmo topology have been studied systematically.[23] Prior to the recent reports of ZJU-280 (SIFSIX-22-Cu, [Cu(tepb)SiF6]) and TIFSIX-Cu-TPB (TIFSIX-6-Cu, [Cu(tepb)TiF6]), fsc-2-SIFSIX ([Cu3(4-(pyridin-4-yl)acrylic acid)4(SiF6)]), CPM-131 ([(TPyP-Fe)Zn(SiF6)]), and its analogues ([(TPyP-M)Cu(NbOF5)], M = Zn, Fe, Ni) were the only HCNs with (4,6)-connected fsc topology, and their sorption properties were found to be driven by coordinatively unsaturated metal centers (UMCs) rather than electrostatics and tight binding sites.[24−28] In fsc-2-SIFSIX, a bifunctional organic linker ligand allows for incorporation of both mononuclear Cu(II) and dinuclear {Cu2} paddle-wheel building blocks into the final structure.[28]CPM-131 (and the related fsx net CPM-132) is constructed using a porphyrin-based metalloligand, and despite the challenges of tuning a porphyrin-based system, it exemplifies HCNs based on a polytopic (used herein to refer to connectivity >2) ligand.[25,26] In 2016, Lusi et al. reported a family of HCNs based on the polytopic linker Tripp (2,4,6-tris(4-pyridyl)pyridine), Tripp-Cu-MFSIX ([[Cu6(Tripp)8](MF6)3(MF6)3]).[29] These structures formed a partially bridged pto-type net but were not found to be permanently porous despite large solvent-accessible voids. A notable feature of this platform is that five distinct inorganic pillar dianions were incorporated into the same structure. More recently, Wu et al. reported an ith-d topology framework, SIFSIX-Cu-TPA ([Cu3(TPA)4(SiF6)3]), using a tritopic linker ligand, TPA (tri(pyridin-4-yl)amine).[30] Our group recently reported the fsc frameworks SIFSIX-22-Zn and SOFOUR-1-Zn, both of which are based on the tepb linker.[31]

The modularity of a porous coordination network (PCN) can be expressed in terms of how many components can be varied independently. HCNs based on two-dimensional nets pillared by MFSIX to form pcu networks are highly modular, having three components that can be varied independently (organic linker ligand, inorganic anionic pillar ligand, and metal cation node).[2] Additionally, there are cases in which interpenetration can also be controlled, for example, in SIFSIX-2-Cu and SIFSIX-2-Cu-i.[4] The resulting drastic effect on the pore size and pore chemistry that results from interpenetration (and therefore properties) offers a fourth variable by which such a platform can be modulated. This level of modularity (four components) is only met or surpassed by platforms that combine mixed linkers and/or extra-framework anions/cations. Most other well-known PCNs, such as those based on oxo-clusters and carboxylate linkers, offer just one or two modular components, limiting the scope of related materials that can be generated and, therefore, the extent to which properties can be “fine-tuned”.

A crystal engineering approach predominantly based on ditopic linker ligands means that most HCNs exhibit nearly cylindrical one-dimensional channels with a high density of tight binding sites to drive their sorption properties. The design of HCNs based on polytopic ligands offers the possibility of new types of channel architectures and new structure–property relationships. In their recent work on ZJU-280 (SIFSIX-22-Cu), Qian and co-workers reported a HUM that is composed of a tetratopic linker ligand in place of the more commonly used ditopic linkers, presenting an opportunity for the development of a new HUM platform for the study of structure–property relationships.[24] We recently reported SIFSIX-22-Zn and SOFOUR-1-Zn using the same tetrapyridyl linker and SiF62– or SO42– pillars, respectively.[31] In the present work, we report a crystal engineering study of fsc HUMs involving substitution of the inorganic pillars in [Zn(tepb)SiF6] (SIFSIX-22-Zn) to afford an additional five members of this platform: [Zn(tepb)TiF6] (TIFSIX-6-Zn), [Zn(tepb)SnF6] (SNFSIX-2-Zn), [Zn(tepb)GeF6] (GEFSIX-4-Zn), [Zn(tepb)ZrF6] (ZRFSIX-3-Zn), and [Zn(tepb)TaF7] (TAFSEVEN-1-Zn).

Experimental Section

Materials and Methods

All reagents and solvents were used as received from vendors. 1H NMR spectroscopy was performed using a JEOL ECX400 spectrometer operating at 400 MHz. Thermal gravimetric analysis (TGA) was performed using a TA Q50 analyzer with a ramp rate of 10.00 °C/min from 25 to 500 °C and nitrogen gas flow of 40 mL/min. Powder X-ray diffraction (PXRD) diffractograms were recorded using a PANalytical X’Pert operated at 40 kV and 40 mA and CuKα radiation (λα = 1.540598 Å) was used for diffraction experiments. Incident beam optics included the Fixed Divergence slit with antiscatter slit PreFIX module, with a 1/8° divergence slit and a 1/4° antiscatter slit, as well as a 10 mm fixed incident beam mask and a Ni-β filter. Data were collected from 5°–40° (2θ) with a step-size of 0.0131303° and a scan time of 30 s per step.

Synthesis of 1,2,4,5-Tetra(4-pyridyl)benzene, tepb

1,2,4,5-Tetra(4-pyridyl)benzene (tepb) was synthesized according to the procedure reported by Chang and Wang.[32] Fe(NO3)3·9H2O (0.161 g, 0.4 mmol), H3PO4 (0.6 mL, 9 mmol), 1,3-bis(4-pyridyl)propane (1.268 g, 6.4 mmol), oxalic acid dihydrate (0.151 g, 1.2 mmol), and water (2 mL) were combined in a Teflon-lined pressure vessel and heated at 180 °C for 48 h. Needle crystals of tepb were manually removed, washed with cold methanol, and dried (yield: ca. 50%). 1H NMR (400 MHz, DMSO-d6): δ = 8.48 (8H, d), 7.61 (2H, s), 7.24 (8H, d).

Synthesis of [Zn(tepb)SiF6], SIFSIX-22-Zn

A solution of ZnSiF6·6H2O (1.0 mg, 0.003 mmol) in 0.4 mL of methanol was put in a narrow glass tube. Methanol (0.2 mL) was carefully layered over this solution to act as a buffer layer before a solution of tepb (1.2 mg, 0.003 mmol) in 0.4 mL methanol was layered over the buffer layer. The tube was left undisturbed for 5 days, at which point colorless block crystals of [Zn(tepb)SiF6]·xMeOH (as-synthesized SIFSIX-22-Zn) were obtained. A larger quantity of SIFSIX-22-Zn was prepared as follows: ZnSiF6·6H2O (41.2 mg, 0.13 mmol) was added to a solution of tepb (77.2 mg, 0.20 mmol) in 16 mL methanol and stirred at room temperature overnight. [Zn(tepb)SiF6]·xMeOH was obtained as a white microcrystalline powder which was isolated by filtration before being washed with methanol and air dried. Yield: 43.0 mg, 56%. CHN analysis calculated for C26H26F6N4O4SiZn (including four interstitial water molecules): C 46.89%, H 3.94%, N 8.41%; experimental: C 46.72%, H 3.12%, N 8.27%.

Synthesis of [Zn(tepb)TiF6], TIFSIX-6-Zn

A solution of Zn(NO3)2·6H2O (0.89 mg, 0.003 mmol) and (NH4)2TiF6 (0.59 mg, 0.003 mmol) in 0.2 mL of water was placed in a narrow glass tube. 1:1 methanol/water (0.4 mL) was carefully layered over this solution to act as a buffer layer before a solution of tepb (1.2 mg, 0.003 mmol) in 0.4 mL methanol was layered over the buffer layer, and the tube was left undisturbed for 3 days, at which point colorless block crystals of [Zn(tepb)TiF6]·xMeOH (as-synthesized TIFSIX-6-Zn) were obtained. A bulk sample of TIFSIX-6-Zn was prepared as follows: Zn(NO3)2·6H2O (77.3 mg, 0.26 mmol) and (NH4)2TiF6 (51.5 mg, 0.26 mmol) in 1.0 mL of water was added to a solution of tepb (154.4 mg, 0.4 mmol) in 30 mL methanol and stirred at room temperature overnight. [Zn(tepb)TiF6]·xMeOH was obtained as a white microcrystalline powder, isolated by filtration, washed with methanol, and air-dried. Yield: 96 mg, 60%. CHN analysis calculated for C26H28F6N4O5TiZn (including five interstitial water molecules): C 44.37%, H 4.01%, N 7.96%; experimental: C 44.37%, H 3.41%, N 8.11%.

Synthesis of [Zn(tepb)GeF6], GEFSIX-4-Zn

A solution of Zn(NO3)2·6H2O (0.89 mg, 0.003 mmol) and (NH4)2GeF6 (0.66 mg, 0.003 mmol) in 0.2 mL of water was placed in a narrow glass tube. 1:1 methanol/water (0.4 mL) was carefully layered over this solution to act as a buffer layer. Finally, a solution of tepb (1.2 mg, 0.003 mmol) in 0.4 mL methanol was layered over the buffer layer, and the tube was left undisturbed for 3 days. Colorless block crystals of [Zn(tepb)GeF6]·xMeOH (as-synthesized GEFSIX-4-Zn) were thereby obtained. A bulk sample of GEFSIX-4-Zn was prepared as follows: Zn(NO3)2·6H2O (77.3 mg, 0.26 mmol) and (NH4)2GeF6 (57.9 mg, 0.26 mmol) in 1.0 mL of water was added to a solution of tepb (154.4 mg, 0.4 mmol) in 30 mL methanol and stirred at room temperature overnight. [Zn(tepb)GeF6]·xMeOH was obtained as a white microcrystalline powder, which was isolated by filtration, washed with methanol, and air-dried. Yield: 108 mg, 65%. CHN analysis calculated for C26H26F6N4O4GeZn (including four interstitial water molecules): C 43.95%, H 3.69%, N 7.87%; experimental: C 43.95%, H 3.24%, N 7.89%.

Synthesis of [Zn(tepb)SnF6], SNFSIX-2-Zn

A solution of Zn(NO3)2·6H2O (0.89 mg, 0.003 mmol) and (NH4)2SnF6 (0.80 mg, 0.003 mmol) in 0.2 mL of water was placed in a narrow glass tube. 1:1 methanol/water (0.4 mL) was carefully layered over this solution to act as a buffer layer. Finally, a solution of tepb (1.2 mg, 0.003 mmol) in 0.4 mL methanol was layered over the buffer layer, and the tube was left undisturbed for 3 days. Colorless block crystals of [Zn(tepb)SnF6]·xMeOH (as-synthesized SNFSIX-2-Zn) were thereby obtained. A bulk sample of SNFSIX-2-Zn was prepared as follows: a solution Zn(NO3)2·6H2O (77.3 mg, 0.26 mmol) and (NH4)2SnF6 (69.4 mg, 0.26 mmol) in 1.0 mL of water was added to a solution of tepb (154.4 mg, 0.4 mmol) in 30 mL methanol and stirred at room temperature overnight. [Zn(tepb)SnF6]·xMeOH was obtained as a white microcrystalline powder, which was isolated by filtration, washed with methanol, and air-dried. Yield: 119 mg, 67%. CHN analysis calculated for C26H34F6N4O8SnZn (including eight interstitial water molecules): C 37.69%, H 4.14%, N 6.76%; experimental: C 37.57%, H 3.20%, N 6.75%.

Synthesis of [Zn(tepb)ZrF6], ZRFSIX-3-Zn

A solution of Zn(NO3)2·6H2O (0.89 mg, 0.003 mmol) and K2ZrF6 (0.85 mg, 0.003 mmol) in 0.2 mL of water was placed in a narrow glass tube. 1:1 methanol/water (0.4 mL) was carefully layered over this solution to act as a buffer layer. Finally, a solution of tepb (1.2 mg, 0.003 mmol) in 0.4 mL methanol was layered over the buffer layer, and the tube was left undisturbed for 3 days. Colorless block crystals of [Zn(tepb)ZrF6]·xMeOH (as-synthesized ZRFSIX-3-Zn) were thereby obtained. A bulk sample of ZRFSIX-3-Zn was prepared as follows: A solution Zn(NO3)2·6H2O (77.3 mg, 0.26 mmol) and K2ZrF6 (73.7 mg, 0.26 mmol) in 10.0 mL of water was added to a solution of tepb (154.4 mg, 0.4 mmol) in 30 mL methanol and stirred at room temperature overnight. [Zn(tepb)ZrF6]·xMeOH was obtained as a white microcrystalline powder, which was isolated by filtration, washed with methanol, and air-dried. Yield: 116 mg, 68%. CHN analysis calculated for C26H28F6N4O5ZrZn (including five interstitial water molecules): C 41.80%, H 3.78%, N 7.50%; experimental: C 41.92%, H 2.91%, N 7.82%.

Synthesis of [Zn(tepb)TaF7], TAFSEVEN-1-Zn

A solution of Zn(NO3)2·6H2O (0.89 mg, 0.003 mmol) and (NH4)2TaF7 (1.05 mg, 0.003 mmol) in 0.2 mL of water was placed in a narrow glass tube. 1:1 methanol/water (0.4 mL) was carefully layered over this solution to act as a buffer layer. Finally, a solution of tepb (1.2 mg, 0.003 mmol) in 0.4 mL methanol was layered over the buffer layer, and the tube was left undisturbed for 3 days. Colorless block crystals of [Zn(tepb)TaF7]·xMeOH (as-synthesized TAFSEVEN-1-Zn) were thereby obtained. A bulk sample of TAFSEVEN-1-Zn was prepared as follows: a solution Zn(NO3)2·6H2O (77.3 mg, 0.26 mmol) and (NH4)2TaF7 (91.0 mg, 0.26 mmol) in 1 mL of water was added to a solution of tepb (154.4 mg, 0.4 mmol) in 30 mL methanol and stirred at room temperature overnight. [Zn(tepb)TaF7]·xMeOH was obtained as a white microcrystalline powder, which was isolated by filtration, washed with methanol, and air-dried. Yield: 113 mg, 57%. CHN analysis calculated for C26H22F6N4O2TaZn (including two interstitial water molecules): C 38.95%, H 2.77%, N 6.99%; experimental: C 38.81%, H 2.25%, N 7.05%.

X-ray Crystallography

Single-crystal X-ray crystallographic data were collected at 298 K on a Bruker D8 Quest diffractometer equipped with a CuKα microfocus source (λ = 1.5406 Å) and Photon 100 detector. Temperature was controlled using a nitrogen flow from Oxford Cryosystems. Data was indexed, integrated and scaled in APEX3.[33] Absorption correction was performed by the multi-scan method using SADABS.[34] Space group determination was performed simultaneously with structure solution using the intrinsic phasing method (SHELXT),[35] and the solution was refined on F2 using SHELXL[36] nonlinear least squares implemented in Olex2 v1.2.10.[37] All nonhydrogen atoms were refined anisotropically and hydrogen atoms bonded to carbon atoms were added at calculated positions and refined using a riding model. Disordered solvents were found in the cavity of all structures. Some of this electron density could be modeled as methanol molecules with partial occupancy; however, refinement was unsatisfactory, and the solvent atomic displacement parameters were unreasonable. The PLATON SQUEEZE[38] routine was performed to account for the electron density of unmodelled solvents, resulting in a more satisfactory refinement. The crystal structure CIF files have been deposited in the Cambridge Crystallographic Data Centre (CCDC: 2151309–2151313).

Gas Sorption Measurements

For gas sorption experiments, ultra-high purity gases were used as received from BOC Gases Ireland: He (99.999%), CO2 (99.995%), C2H2 (98.5%), N2 (99.998%), C2H4 (99.92%), and C2H6 (99.0%). Adsorption isotherm experiments (up to 1 bar) for 195 K CO2 were performed on a Micromeritics Tristar II 3030. A Micromeritics 3Flex surface area and pore size analyzer 3500 was used for collecting the 273 and 298 K sorption isotherms for all gases. The low temperature of 195 K was maintained using a dry ice-acetone mixture. Bath temperatures of 273 and 298 K were precisely controlled with a Julabo ME (v.2) recirculating control system containing a mixture of ethylene glycol and water. Prior to experiments, SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn were activated on a Smart VacPrep using dynamic vacuum and heating for 24 h at 333 K.

Results and Discussion

Structural Description

TIFSIX-6-Zn crystallized in the centrosymmetric orthorhombic space group Pmma, SIFSIX-22-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, and TAFSEVEN-1-Zn crystallized in the centrosymmetric orthorhombic space group Cmma while ZRFSIX-3-Zn crystallized in the centrosymmetric orthorhombic space group Pmmm. All six structures are comprised as expected: octahedral Zn2+ ions coordinated to four pyridyl moieties of tepb ligands in their equatorial positions and bridging MF62– (M = Si(IV), Ti(IV), Ge(IV), Sn(IV), and Zr(IV)) or TaF72– anions in their axial positions. The Zn2+ ions and tepb ligands formed two-dimensional layers pillared by the MF62– or TaF72– anions to generate three-dimensional 4,6-connected fsc topology networks (Figure ).

Figure 1

Representations of (a) tetra(4-pyridyl)benzene (tepb) ligand and Zn2+ metal center; (b) MFSIX and TAFSEVEN pillars; and (c) fsc network [Zn(tepb)MF6] viewed along the c (above) and a (below) crystallographic axes.

The Zn–F distance in TAFSEVEN-1-Zn is 2.040(2) Å, which lies within the lower quintile (2.061 Å) of the mean distance of 2.108 Å (st. dev. = 0.059 Å) as per the Cambridge Structural Database[39] (CSD, v. 5.41 + 3 updates, see the SI for search parameters), while the Zn–F distances for SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, and ZRFSIX-3-Zn lie in the second quintile (2.092 Å). The TAFSEVEN-1-Zn Zn–N distance range of 2.176(3) Å lies within the upper quintile (2.157 Å) of the mean distance of 2.138 Å (st. dev. = 0.020 Å) as per the same search query. Zn–N distances for GEFSIX-4-Zn and SNFSIX-2-Zn lie in the third quintile (2.145 Å) while the Zn–N distances for SIFSIX-22-Zn, TIFSIX-6-Zn, and ZRFSIX-3-Zn lie in the fourth quintile (2.157 Å).

The pyridine rings of the tepb ligand adopt a propeller-type arrangement about the central benzene ring with rings para to each other being co-planar in TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn (Figure ). The torsion angles of the pyridine ring about the Zn2+ ions are given in Figures S1–S4 and range from 52.3(2)° to 70.11(5)°. The pyridyl rings are arranged in a propeller-like conformation about the zinc(II) ion. The two-dimensional Zn2+-tepb layers are pillared by the MF62– and TaF72– pillars such that the central aromatic rings of the tepb ligands are eclipsed and coplanar when viewed down the crystallographic a-axis for SIFSIX-22-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn and the crystallographic b-axis for TIFSIX-6-Zn. In the case of TIFSIX-6-Zn and ZRFSIX-3-Zn, the fluorine atoms are modeled as disordered over two positions with each position being eclipsed with the fluorine atoms above and below the plane. For SIFSIX-22-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, and TAFSEVEN-1-Zn, the equatorial fluorine atoms of the MF2– pillar are not eclipsed with GEFSIX-4-Zn having the largest pillar rotation angle of 55.0(2)°. In the case of SNFSIX-2-Zn, the equatorial fluorine atoms of the SnF62– pillar are modeled as disordered over two positions with each position exhibiting noticeably different pillar rotation angles: 4.9(5)° and 23.0(5)°. In the two-dimensional zinc(II)-tepb layers, there are two distinct windows (Figure S6), the sizes of which are roughly consistent across all six compounds as the window geometry is independent of the anionic pillar: narrow, square windows where the pyridyl rings are ortho relative to one another; larger rectangular windows when pyridyl rings are meta relative to one another.

Figure 2

Formula units in SIFSIX-22-Zn, TIFSIX-6-Zn, SNFSIX-2-Zn, GEFSIX-4-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn. Thermal ellipsoids are shown at a probability level of 50%.

Pore structures were calculated from the crystallographic data of SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn (Poreblazer v4.0, Tables S7, S8). The pore volume per formula unit ranges from 90.49 Å3 for TAFSEVEN-1-Zn to 135.20 Å3 for ZRFSIX-3-Zn. The maximum pore diameters vary from 4.71 to 5.92 Å (Poreblazer v4.0), whereas the limiting pore diameters range from 3.18 to 3.72 Å.[40] The constricted pore region is a result of the narrow pore windows occurring in the Zn2+-tepb layers while the larger pore cavity exists between these two-dimensional layers. This degree of pore constriction is high compared to other HCNs such as mmo topology networks and Tripp-Cu-SIFSIX despite their unique network architectures.[23,29] The highly constricted pores present in SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn are comparable to SIFSIX-18-M and DICRO-6-Co-i among the pcu topology HCNs which feature constricted pore environments (Figure ). For both of the aforementioned materials, the pore constriction arises from the distortion of the one-dimensional metal-pillar-metal chain from a linear to zig-zag arrangement. In SIFSIX-18-M, this is due to the shape of the ligand, whereas in DICRO-6-Co-i it results from the nonlinear geometry of the pillar.[13,41] In contrast, the constriction of pores in SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn arises solely from the dimensions of the Zn2+-tepb layers.

Figure 3

Accessible void surface calculated for SIFSIX-3-Zn viewed along the crystallographic (a) a- and (b) c-directions. Representations of the accessible void surface calculated for SIFSIX-22-Zn viewed along the crystallographic (c) a- and (d) c-directions. (e) Plot of crystallographically determined maximum and limiting pore diameters in representative HCN materials (red dashed line: idealized cylindrical pores; blue dashed line: the ratio of maximum pore diameter to pore limiting diameter in SIFSIX-22-Zn; see the SI for tabulation).

Characterization

Bulk samples of each compound were synthesized by room-temperature slurry methods and characterized by PXRD and TGA (Figures S9–S14). PXRD patterns collected after immersion under MeOH for 1 week revealed that all samples had retained their crystal structures. PXRD patterns collected after exposure to accelerated stability test conditions at 75% R.H. and 40 °C after 1 day and 1 week revealed that SIFSIX-22-Zn underwent hydrolysis and an associated phase change within 1 day, while GEFSIX-4-Zn underwent a phase change between 1 day and 1 week. No change was observed in the PXRD patterns of TIFSIX-6-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn after 1 week, indicating that these pillars provide enhanced hydrolytic stability.

Gas Sorption

After activating as-synthesized bulk samples of each HUM, CO2 sorption isotherms were measured at 195 K to determine their textural characteristics. In all six materials, type I isotherms with steep uptake at low pressure were observed with saturation uptakes of 150–200 cm3 g–1 (Figure a). Brunauer–Emmett–Teller (BET) surface areas of 387.2, 396.8, 700.0, 615.4, 625.9, and 627.5 m2 g–1 were determined for SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn, respectively. The Horvath–Kawazoe plots obtained from 195 K CO2 data revealed a narrow range of maximum pore widths between 3.7 and 4.1 Å (Figure b). These values experimentally confirm the categorization of these materials as ultramicroporous and lie within the range of crystallographically determined pore dimensions (Tables S7–S9). N2 sorption isotherms were also collected and afforded BET surface area values of 414.3, 478.1, 925.1, 798.1, 1062.0, and 652.9 m2 g–1 for SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn, respectively (Figures S16–S21). The variations between saturation CO2 and N2 uptakes are attributed to the differing sizes and quadrupole moments of N2 and CO2 and their interactions with the differing electrostatics of the surfaces of each adsorbent.[42]

Figure 4

(a) 195 K CO2 sorption isotherms on SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn; (b) Horvath–Kawazoe pore-size distribution plots obtained using 195 K CO2 isotherms. 298 K sorption isotherms of (c) CO2, (d) C2H2, (e) C2H4, and (f) C2H6 on SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn.

Following these observations, we investigated the room-temperature sorption properties of each HUM toward CO2 and C2 hydrocarbons. The CO2, C2H2, C2H4, and C2H6 sorption isotherms for SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, and ZRFSIX-3-Zn are well-defined Langmuir-type profiles (Figure c–f). PXRD data collected after sorption experiments support the apparent reversibility of the isotherms and no loss of crystallinity was observed (Figures S9–S14). CO2 uptakes at 298 K varied from a maximum of 99 cm3 g–1 in GEFSIX-4-Zn to a minimum of 81 cm3 g–1 in ZRFSIX-3-Zn. Similarly, 298 K C2H2 uptakes ranged from 127 cm3 g–1 in TIFSIX-6-Zn to 102 cm3 g–1 in ZRFSIX-3-Zn. C2H4 and C2H6 isotherms exhibited similar profiles, with lower uptakes overall vs. C2H2. The only deviation from ideal Langmuir-type profiles was seen in the CO2 isotherm of TAFSEVEN-1-Zn, in which a minor inflection occurred at ca. 0.55 bar and 298 K. We attribute this anomaly to the five equatorial fluorides in the TaF72– pillar leading to a distinct electrostatic distribution vs MF62– pillars, thereby impacting sorption through F···HAr contacts to the tepb ligands by altering ligand conformation and pore dimensions.

Isosteric heats of sorption (Qst) were calculated for CO2 and C2 gases for all six adsorbents (Figure a,b). Low loading Qst values for CO2 varied from 43.3 and 42.6 kJ mol–1 for SNFSIX-2-Zn and ZRFSIX-3-Zn, respectively, to 30.4 and 24.7 kJ mol–1 for TIFSIX-6-Zn and SIFSIX-22-Zn, respectively. Low loading Qst values for C2H2 were determined to be relatively higher with a narrower range, from 44.8 kJ mol–1 for TIFSIX-6-Zn and GEFSIX-4-Zn to 36.5 kJ mol–1 for SIFSIX-22-Zn. In contrast, Qst values for C2H4 and C2H6 range from 33.3 to 31.6 kJ mol–1 for C2H4 and 32.7 to 31.1 kJ mol–1 for C2H6 (Figures S22, S23). Overall, the affinity for C2H2 was highest, encouraging us to evaluate the selectivity of the six HUMs in the context of C2H2/CO2 and C2H2/C2H4 separations.

Figure 5

Isosteric heats of adsorption of (a) CO2 and (b) C2H2 on SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn. Ideal adsorbed solution theory (IAST) selectivity determined for (c) 1:1 C2H2/CO2 (SAC) and (d) 1:99 C2H2/C2H4 (SAE) for SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, ZRFSIX-3-Zn, and TAFSEVEN-1-Zn. Comparative plots of leading HCNs with respect to (e) SAC and (f) SAE versus C2H2 uptake.

Calculations conducted using ideal adsorbed solution theory (IAST) indicate that C2H2/CO2 (1:1) selectivities (SAC) vary between 6.5 (SIFSIX-22-Zn) and 3.8 (TAFSEVEN-1-Zn) at 1 bar. C2H2/C2H4 (1:99) selectivities (SAE) at 1 bar vary between 18.7 (SIFSIX-22-Zn) and 4.7 (TAFSEVEN-1-Zn) (Figure c,d). Perhaps most notable is that, despite the absence of any apparent correlation between pure component Qst values for adsorbents and different gases, SAC and SAE values follow a clear trend, that is, SIFSIX-22-Zn > SNFSIX-2-Zn > TIFSIX-6-Zn > GEFSIX-4-Zn > ZRFSIX-3-Zn > TAFSEVEN-1-Zn. This correlation suggests that the identity of inorganic anions is responsible for the varying affinity toward acetylene in these HUMs and that the sorption properties are impacted by incorporation of different pillars. No correlation between SAC and SAE values with the electronegativity of the central atom of the anion was evident, suggesting a need for calculation of pore surface charges and in-depth detailed computational studies on this system to fully elucidate the observed affinities toward C2H2.

The calculated selectivity values of these adsorbents are moderate but when viewed together with their relatively high uptakes, it is apparent that these fsc networks address the trade-off between selectivity and uptake (Figure e,f). When compared to other C2H2 selective sorbents, SIFSIX-22-Zn and TIFSIX-6-Zn show a rare combination of strong selectivity and high uptake, indicating that further exploration of this platform of materials has the potential to produce adsorbents with strong overall performance.

Conclusions

We report herein the highly modular fsc topology HUM platform which enabled us to explore the effect of changing inorganic pillar on gas sorption properties. The use of SiF62–, TiF62–, GeF62–, SnF62– ZrF62–, and TaF72– in combination with Zn2+ and the tetratopic tepb ligand afforded a family of HUMs: SIFSIX-22-Zn, TIFSIX-6-Zn, GEFSIX-4-Zn, SNFSIX-2-Zn, and ZRFSIX-3-Zn and the first TaF7-pillared HUM, TAFSEVEN-1-Zn The crystal structures were determined by single-crystal X-ray diffraction and enabled systematic studies of structure–property relationships. Each framework features narrow pore windows, yet significant pore cavities between the zinc(II)-tepb layers. This is unusual in HUMs and presents a general strategy that could address the trade-off between uptake and selectivity that is common in adsorbents. Variation of the inorganic pillar resulted in a trend in relative C2H2 affinity as follows: SIFSIX-22-Zn > SNFSIX-2-Zn > TIFSIX-6-Zn > GEFSIX-4-Zn > ZRFSIX-3-Zn > TAFSEVEN-1-Zn. Overall, this work highlights the modular nature of the fsc HUM platform and that substitution of the inorganic pillar impacts structure–property relationships.